A Linear Voltage Regulator (hereinafter “regulator”) is a DC voltage regulator circuit based on an active element which is operating in its linear region. It accepts a DC voltage at its input and provides a regulated DC voltage at its output. It is a basic building block of a power supply which provides power to electronic devices (the “load”). The most common designs use feedback circuits to compare the output to a reference voltage and apply correction, albeit with a time lag, to keep the output voltage constant. Many such regulators have been developed; they are widely available in integrated circuit (“IC”) form, and can be built using discrete components.
In many applications, for instance when it is used to power devices that process AC signals, in addition to providing a regulated DC voltage, the regulator also functions as a source of AC current on demand. Ideally, the output impedance of the AC current source would be resistive over the frequency range of use. This can be examined by plotting the regulator's output impedance and impedance phase vs. frequency. The typical pattern found is that of an output impedance with some small finite resistance and then rising with increasing frequency (FIG. 1-101), a basically inductive characteristic as verified by the impedance phase (FIG. 1-102). It can be modeled (FIG. 3-301) as a resistance (Rout) in series with an inductance (Lout). This output impedance profile is not ideal for an AC current source and should be compensated for.
Additionally, under some conditions, the distribution of the regulator's output impedance vs. frequency is not constant, but is significantly affected by the amount of quiescent (or “idle”) current being drawn by the load. Refer to FIG. 2, in which plots of output impedance vs. frequency at various idle currents for a common regulator are graphed. 201 graphs 10 mA, 202 graphs 15 mA, 203 graphs 30 mA, 204 graphs 40 mA, 205 graphs 50 mA, and 206 graphs 100 mA idle currents. Starting from a no-load condition, as the idle current is increased, the regulator output impedance trends rapidly lower, to a point where further current increases result in little impedance change, marking the low point of a “stable region.” For the regulator shown, this point is around 40-50 mA. A similar phenomena occurs as the load current begins to approach the regulator's output current design limits, marking the high point of the stable region. From these graphs, it can be seen that operating a regulator at idle currents outside of its stable region will result in significant impedance modulation as the load draws AC current, adding error to the system.
Industry standard practice is to place capacitance from the regulator output to the zero volts reference (aka “common” or “ground”), partly to counteract the inductive regulator output impedance, but primarily to filter the regulator's output noise. Some regulators require a minimum, or specified range of, output capacitance value for stable operation. And some regulators require specific values of Equivalent Series Resistance (“ESR”) of the output capacitor for stable operation. While simply adding capacitance to the output does lower the output noise and overall average output impedance, this approach has a significant drawback. It forms a resonant circuit with the regulator Lout, creating a nonuniform impedance (FIG. 1-103) and impedance phase (FIG. 1-104) characteristic.
Two prior techniques have been used to counter this resonance. A common technique is to place a resistor (typically 1 Ohm and greater) in series with the regulator output, between the regulator output and the output capacitor. The large resistance swamps the Lout and forms an RC filter with the output capacitor. While this is effective for improving the output noise filtering, it has several drawbacks from an impedance perspective. AC current drawn through the large resistor induces proportionally more AC ripple on the DC voltage. The resistor limits the maximum current which can be delivered to the load. And the output impedance and impedance phase, which was inductive, is now capacitive, and still very nonuniform.
Another technique (Calex) adds sufficient resistance (typically 100 mOhms or greater) in series with the output capacitor, between output capacitor and ground, to damp the resonance. This smoothes the impedance at and above resonance. But it has no effect on the frequency region below the resonance, leaving intact the inductive characteristic, and a nonuniform phase characteristic as well.
While not technically an output impedance compensation circuit, it is worth noting that some high-performance discrete regulators have been designed (Jung et al) that significantly lower the overall output impedance and voltage noise. But they still have an inductive output impedance characteristic with the attendant phase shift.
In none of the above cases was there an intent to make the regulator output impedance and impedance phase uniform, or to coordinate the impedance compensation circuit with the idle current effects. Therefore, it would be desirable to provide a method to accomplish this, in a form that can be applied to any regulator with an inductive output impedance characteristic.